Advantages of 18 inches over 12 inches

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Talyn
Post subject: Advantages of 18 inches over 12 inches
Posted: Sun Jun 13, 2010 9:58 am
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I am aiming to get a larger scope in the not-too-distant future, maybe early next year, depending on how the saving for it goes. I currently use a 12" Dobsonian but I am hoping to get an 18".

In real terms, how much extra light grasp will I get from an 18" over a 12"?

And, although I am not going for a 16" (if I was, I'd just get a Meade LB and have done with it), what advantage is there in a 16" over a 12"? There is a thread over on CN about this, but there seems to be differing answers, and the inevitable arguments, so I am curious.
Is there much difference between a 16" and an 18"?


Last edited by Talyn on Mon Jun 14, 2010 5:36 am, edited 1 time in total.

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Erik
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Posted: Sun Jun 13, 2010 11:55 am
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There's a big difference between 12" and 16". Galaxies are much, much brighter in a 16". There's less difference between a 16" and an 18"; in fact, I had the chance to compare my 16" many times to an 18" that CNer Craig C had. It was very close- much of the time it was difficult to see a difference. However, I would add that my mirror was freshly coated at the time while his had been coated several years earlier. Still, with the size difference, I was happier with a 16", if I were to move up, it would have to be to something in the 20-24" range. And that would mean I'd have to give up any aspect of portability.

From a 12", I'd go to an 18" if you don't plan on transporting it and if budget isn't an issue. Otherwise, a 16" will show you a noticeable difference.

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snorkler
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Posted: Sun Jun 13, 2010 4:15 pm
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I agree with Erik. I can see more difference between 10" and 12.5" scopes than I can between 16 and 18" scopes. Yet, everything I've seen in my buddy's 12.5", I can also see in my 10". His scope just makes them more obvious, and I have to try harder with the 10". And there's less difference between 16" and 18". Aperture always wins, so once you're in that range, go with the 18".

I'm probably going to sell my 18" Nightsky, if you're interested. It has a John Hall (Pegasus) mirror with a .992 Strehl ratio! I haven't used it since TSP 2009, so it seems a crime for me to sit on it.

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Chopin
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Posted: Sun Jun 13, 2010 6:30 pm
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Talyn, with regards to both photography and visual observation, a minimum factor increase of 1.4x is always the magic number. This not only relates to aperture but also magnification, making a good factor for eyepiece spacing as well. So, ultimately you want the surface area to increase by at least 1.4x to notice a real difference.

The surface area of a 12" scope is 113 in?. A 16" scope is 201 in?. 201/113=1.8. Big difference.
The surface area of a 12" scope is 113 in?. A 18" scope is 254 in?. 254/113=2.3. Big difference.
The surface area of a 16" scope is 201 in?. A 18" scope is 254 in?. 254/201=1.2. Small difference.

FWIW, I've owned a 12" for 5 years now, so I am intimately familiar with what it can do. Looking through a 14" SCT doesn't blow my socks off, but a 17.5" dob does.

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snorkler
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Posted: Sun Jun 13, 2010 6:45 pm
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In case anyone's feeling sorry for me, I'm in black skies with 25", 17.5", and 13.1" Dobs, and a C14 on an AP1200 for the next month.

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Ah, but I was so much older then.
I'm younger than that now
Bob Dylan

[ img ]
49.6 mpg avg over 38,000 miles. 177% of '08 EPA
Best flat drive 94.5 mpg for 9.8 mi
Best tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg


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Chopin
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Posted: Sun Jun 13, 2010 7:16 pm
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Feeling sorry for you...hating your guts...it's all the same, right? :razz:

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Talyn
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Posted: Mon Jun 14, 2010 7:40 am
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Re. 16" over 12", that's as much as I thought. There's a guy on CN who swears that a 16" offers no advantage over 12" and I didn't think that was right.

Snorkler, how much do you want for the 18"?

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Kawx4d
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Posted: Mon Jun 14, 2010 9:10 am
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Definitely go with the 18" in this case since you are moving up from a 12. Not only brightness but resolution will be better, and the weight and bulk difference between the 16 and 18 isn't that much.

In addition you will go a full magnitude deeper for every 254% increase in surface area. In this case the 18 is about 225% so you are getting .89 magnitudes deeper over your 12 vs. about 0.7 if you went with the 16. The amount of stars visible from one magnitude to the next are exponential so it opens up a hell of a lot more sky at the eyepiece than your 12. The 18 has an additional .2 mags over the 16, which wouldn't blow you away, but it could make the difference in detecting faint galaxies or seeing a little more at the threshold.

The guy on CN who says a 16 doesn't offer any real advantage over a 12 is probably looking, rather than observing. There is a noticeable difference. It might not be the knock-you-socks-off difference you get from a 12 to a 20, but there is a noticeable increase in the amount of information available to you in the 16.

In this case, I think the 18 would be a great way to go for you if you have no issues with moving the scope. Good luck!

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snorkler
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Posted: Mon Jun 14, 2010 6:06 pm
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Faith,

Asking $4,500 USD. It has DSCs and Servocat installed, but I can't find the Argo-Navis controller. Groundboard driven by Servocat has one flat spot that may need rebuilding.

It's f/4.2, nice and short, so I never have to climb more than 2 steps on the ladder.

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Ah, but I was so much older then.
I'm younger than that now
Bob Dylan

[ img ]
49.6 mpg avg over 38,000 miles. 177% of '08 EPA
Best flat drive 94.5 mpg for 9.8 mi
Best tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg


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Talyn
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Posted: Wed Jun 16, 2010 11:01 am
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That's quite a bit more than the one I can buy over here, which does not include shipping and import duty (although I am not sure that a second hand scope would attract import duty - although knowing UK Customs, that's entirely possible).

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snorkler
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Posted: Wed Jun 16, 2010 2:22 pm
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Glad to hear you can get a scope like that so reasonably. I think of UK optics prices as typically the same numerical value as our US dollar price, but in Pounds, which makes your cost ~70% higher.

_________________

Ah, but I was so much older then.
I'm younger than that now
Bob Dylan

[ img ]
49.6 mpg avg over 38,000 miles. 177% of '08 EPA
Best flat drive 94.5 mpg for 9.8 mi
Best tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg


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Talyn
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Posted: Thu Jun 17, 2010 6:45 am
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The downside is we do not have the same amount of choice you do in the States. We have one specialised scope maker, David Lukehurst, whose scopes are supposedly very good (and who I am going to go to for my scope) plus the usual mass-produced stuff such as Meade Lightbridges, Orion (USA) and Revelation (GSO). We have Orion UK, but all their Dobs are solid tubes, they don't do an 18" in any case and the one truss tube they do is a 20", which is nearly ?6000 and out of my price range.

We can get Obsessions, etc, and there are a few kicking round over here, but shipping plus import duties put the price up by over ?1000.

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"Well science knows it doesn't know everything, otherwise it would stop. But just because science doesn't know everything, doesn't mean you can fill in the gaps with whatever fairytale most appeals to you." Dara O'Briain, Irish comedian.


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